Human T cell response to viral protein antigens. This project was started to determine the role of the putative HBsAg fusion peptide in the immune response to HBsAg vaccine. As a first step, we have made a series of DNA constructs to determine whether the HBsAg sequence is indeed a functional fusion peptide. These constructs code for a hybrid protein containing the CD4 receptor binding region of HIV-1 gp120, followed by the putative fusion peptide of HBsAg in place of the normal HIV-1 fusion peptide. We have tested the constructs for expression of protein. After optimizing conditions for expression, we will express the protein in a monolayer of cells with CD4 on their surface. In pilot experiments, expression of gp120 with the HIV-1 fusion peptide on these monolayer cells has caused syncytium formation. The new constructs will then be tested in the same way: these include the fusion peptides of HBsAg, duck surface antigen, and influenza hemagglutinin, as well as the HBsAg sequence with a point mutation, which should abrogate membrane fusion. If successful, we will test the effect of the fusion peptide on intracellular trafficking and on antigen processing and presentation. Our hypothesis is that the fusion peptide may enable the viral protein to enter additional processing pathways and may contribute to the immunogenicity of HBsAg by generating a new set of antigenic peptides. These results may be important to naked DNA vaccines, where the goal is to express viral proteins endogenously, in order to elicit MHC class I and class II specific T cell responses.